Exobiology in the Solar System & The Search for Life on Mars - ESA

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large chaotic areas (that produced <strong>the</strong> large outflow channels) are created by this type of mechanism. Ow<strong>in</strong>g to <strong>the</strong> depth of <strong>the</strong> permafrost, <strong>the</strong> <strong>in</strong>vestigation of this potentially lifebear<strong>in</strong>g environment will probably be postponed until <strong>the</strong> human exploration of Mars. However, <strong>the</strong>re are areas at high latitudes where <strong>the</strong> permafrost is thought to exist at very shallow depths. Moreover, steep and high cliffs occur <strong>in</strong> several areas of Mars, and <strong>the</strong>y might expose a portion of <strong>the</strong> permafrost. Current concepts <strong>for</strong> search<strong>in</strong>g <strong>for</strong> microbial fossils on Mars are strongly focused on f<strong>in</strong>d<strong>in</strong>g <strong>the</strong> rema<strong>in</strong>s of <strong>for</strong>mer surface-bound microbial activity <strong>in</strong> sediments and hot spr<strong>in</strong>g environments. While it has become accepted that subsurface life is presently active on Earth to a depth of several km at some places (e.g. Stevens & McK<strong>in</strong>ley, 1995; Stevens 1997), barely any fossil evidence of this has been reported so far, with <strong>the</strong> notable exception of a paper by Kretzschmar (1982), although <strong>the</strong>re have been scattered reports <strong>in</strong> <strong>the</strong> literature s<strong>in</strong>ce 1782. Recent work has shown that subsurface microbialites similar to those reported by Kretzschmar (1982) are common <strong>in</strong> terrestrial samples from numerous subsurface environments (Hofmann & Farmer, 1997). This work is <strong>in</strong> progress and approximately 100 sites have so far been identified. Geological environments conta<strong>in</strong><strong>in</strong>g such suspected fossil rema<strong>in</strong>s <strong>in</strong>clude • sites of low-T hydro<strong>the</strong>rmal alteration of volcanic rocks (dom<strong>in</strong>ant); • oxidation of sulphide ore deposits (quite common); • hydro<strong>the</strong>rmal ve<strong>in</strong>-type ore deposits (rare); • silica deposits associated with serpent<strong>in</strong>isation of ultra-basic rocks (rare). <strong>The</strong> depth of emplacement of filamentous microstructures of probable microbial orig<strong>in</strong> is unknown <strong>in</strong> most cases, but reaches 400-800 m <strong>in</strong> at least three of <strong>the</strong> betterstudied sites. <strong>The</strong>re is a grow<strong>in</strong>g body of evidence that subsurface microbial activity commonly results <strong>in</strong> macroscopic structures similar to surface microbialites (stromatolites). Many of <strong>the</strong> hi<strong>the</strong>rto recognised possibly microbial structures <strong>in</strong> such environments are enclosed <strong>in</strong> microcrystall<strong>in</strong>e silica varieties (chalcedony, agate). <strong>The</strong> recent identification of agates as products of post-impact hydrous alteration <strong>in</strong> an impact melt of a F<strong>in</strong>nish crater (K<strong>in</strong>nunen & L<strong>in</strong>dqvist, 1998) makes it likely that similar silica varieties do exist on Mars, s<strong>in</strong>ce cool<strong>in</strong>g impact melts <strong>in</strong>teract<strong>in</strong>g with groundwater must have been common. <strong>The</strong> likelihood of subsurface microbial fossils on Mars is significant and a subsurface biosphere can be expected to be relatively more important on Mars than on Earth because of <strong>the</strong> <strong>in</strong>hospitality of <strong>the</strong> martian surface. By consider<strong>in</strong>g not only <strong>the</strong> surface-bound but also <strong>the</strong> subsurface microbial fossils <strong>in</strong> <strong>the</strong> process of evaluat<strong>in</strong>g land<strong>in</strong>g sites, target selection <strong>for</strong> sampl<strong>in</strong>g, imag<strong>in</strong>g etc enlarges <strong>the</strong> selection of available possible <strong>in</strong>terest<strong>in</strong>g sites and materials. A fractured or vesicular basalt, <strong>for</strong> example, would be considered of low <strong>in</strong>terest <strong>in</strong> exist<strong>in</strong>g strategies focused on sediments and <strong>the</strong>rmal spr<strong>in</strong>g deposits, but may never<strong>the</strong>less harbour rich subsurface fossil rema<strong>in</strong>s. Reconstruct<strong>in</strong>g <strong>the</strong> climatic and environmental changes of Mars should be done through a detailed reconstruction of its geological history. Water is <strong>the</strong> most important factor among <strong>the</strong> variables that shaped <strong>the</strong> planet, and knowledge of <strong>the</strong> hydrological cycle and long-term variations of <strong>the</strong> environment are based on geological analysis. Of course, <strong>the</strong>re are serious disagreements on <strong>the</strong> reconstruction of <strong>the</strong> geologic history but, basically, two models are proposed. <strong>The</strong> first model proposes that <strong>the</strong> climate be<strong>for</strong>e 3.5 Gyr ago was warmer and wetter. A thicker atmosphere was able to susta<strong>in</strong> a complex climatic and hydrological cycle. In this scenario, ra<strong>in</strong>fall played an important role. Precipitations, <strong>in</strong> fact, were able to degrade a large part of <strong>the</strong> geological features (mostly impact craters). team I: exobiology and <strong>the</strong> mars surface environment/II.4 II.4.2 General Remarks on Subsurface Microbial Fossils II.4.3 Climatic and Environmental Models 115
SP-1231 Fig. II.4.3/1. Ages of several geological units and features. Several events <strong>in</strong>volv<strong>in</strong>g large amounts of water are ra<strong>the</strong>r young. 116 Complex fluvial patterns supplied by large-scale underground water circulation, <strong>in</strong> turn supplied by atmospheric water, <strong>for</strong>med <strong>in</strong>tricate fluvial systems (channel network). <strong>The</strong> subsurface circulation of water supplied hydro<strong>the</strong>rmal systems. Water was present later <strong>in</strong> martian history, but only sporadically and <strong>for</strong> very short periods. <strong>Life</strong>, accord<strong>in</strong>g to this model, developed and was ext<strong>in</strong>guished with<strong>in</strong> <strong>the</strong> first warmer and wetter epoch. This warm, wet model at an early epoch is based on two geological observations: 1) <strong>the</strong> degradation of <strong>the</strong> craters on <strong>the</strong> older surfaces ceased abruptly about 3.5 Gyr ago; 2) <strong>the</strong> valley networks occur on old (older than 3.5 Gyr) surfaces. This model has several shortcom<strong>in</strong>gs. Dat<strong>in</strong>g <strong>the</strong> crater degradation is quite complex and somewhat tautological because crater morphologies are be<strong>in</strong>g dated by crater count<strong>in</strong>g. <strong>The</strong> age of <strong>the</strong> valley network cannot be achieved by def<strong>in</strong><strong>in</strong>g <strong>the</strong> age of <strong>the</strong> unit where <strong>the</strong> channels occur. <strong>The</strong> surfaces can be much older, as happens on Earth, where today’s rivers cut <strong>in</strong>to even Precambrian geological units. However, it is quite probable that erosion by flow<strong>in</strong>g water played a major role <strong>in</strong> <strong>the</strong> degradation of <strong>the</strong> Noachian craters and o<strong>the</strong>r features (Carr, 1983) and this extensive degradation could be <strong>the</strong> product of climatic conditions that could susta<strong>in</strong> a thick and warm atmosphere. <strong>The</strong> second model suggests that several episodes of wetter and warmer climate occurred dur<strong>in</strong>g martian history. Several features <strong>in</strong>dicative of liquid water are scattered over <strong>the</strong> surface. Outflow channels are a remarkable example, but <strong>the</strong>y can be due to <strong>the</strong> dramatic and episodic releases of water at <strong>the</strong> surface and <strong>the</strong>ir lives are ra<strong>the</strong>r short, although <strong>in</strong>tense. <strong>The</strong> important question is <strong>the</strong> fate of <strong>the</strong> water <strong>in</strong> <strong>the</strong> outflow channels. An old hypo<strong>the</strong>sis suggests that <strong>the</strong> water rema<strong>in</strong>s briefly at <strong>the</strong> surface be<strong>for</strong>e sublimat<strong>in</strong>g <strong>in</strong>to <strong>the</strong> atmosphere or percolat<strong>in</strong>g below ground. However, <strong>the</strong>re is evidence of extensive coastl<strong>in</strong>es at <strong>the</strong> border of <strong>the</strong> nor<strong>the</strong>rn lowlands where <strong>the</strong> Valles flow (Parker et al., 1989). This supports <strong>the</strong> idea of a large ocean <strong>in</strong> <strong>the</strong> nor<strong>the</strong>rn hemisphere (Baker et al., 1991). This nor<strong>the</strong>rn ocean could have been active
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SP-1231 Exobiology
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Cover Fossil coccoid bacteria, 1 µ
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4 I.5 Potential Non-Martian Sites <
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6 6.2 Imaging of F
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The Exobio
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pyrolysis, or similar techniques, f
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Ignasi Casanova, Institute
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I AN EXOBIOLOGICAL VIEW OF THE SOLA
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SP-1231 18 surface pressure of CO 2
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SP-1231 20 10 bar befor</st
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SP-1231 22 kilometres in</s
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SP-1231 24 Laboratory Investigation
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I.3 Limits of Life
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temperatures of 95-106ºC. This sug
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(mainly found <str
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cannot exclude fin
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Responses to Cosmic Radiation <stro
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acid to identify the</stron
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Horneck, G. (1993). Responses of Ba
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SP-1231 Fig. I.4.2.2/1. Size distri
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SP-1231 Fig. I.4.2.2/4. Evaporite w
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SP-1231 Fig. I.4.2.3/1A (left). Net
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SP-1231 48 A detailed chemical anal
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SP-1231 Fig. I.4.3.1.1/1. Scheme of
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SP-1231 Fig. I.4.3.1.2/1. A: Compar
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SP-1231 54 I.4.3.2.1 Sedimentary Or
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SP-1231 56 The iso
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SP-1231 Fig. I.4.3.2.3/1. Cholester
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SP-1231 60 References regard to non
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Page 61 and 62: SP-1231 64 ities in</strong
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Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
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Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
Page 77 and 78: II.1 Introduction The</stro
Page 79 and 80: II.2 The Planet Ma
Page 81 and 82: cover the range 4.
Page 83 and 84: The depletion of c
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Page 87 and 88: plate tectonic evidence has been ob
Page 89 and 90: Table II.2.7.4/1. Potential Mars <s
Page 91 and 92: II.3 The Martian M
Page 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
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Page 101 and 102: Anders, E. (1989). Prebiotic organi
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Page 115 and 116: SP-1231 120 II.4.5 The</str
Page 117 and 118: SP-1231 122 II.4.6 Rovers and Drill
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Page 123 and 124: SP-1231 128 Sharp, M., Parkes, J.,
Page 125 and 126: SP-1231 Fig. II.5.1/1. The<
Page 127 and 128: SP-1231 132 II.5.3 Isotopic Analysi
Page 129 and 130: SP-1231 134 chemical composition de
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Page 133 and 134: SP-1231 138 optical microscope. Bot
Page 135 and 136: SP-1231 140 protons of discrete ene
Page 137 and 138: SP-1231 142 atures should be per<st
Page 139 and 140: SP-1231 Fig. II.5.7.7/1. Example of
Page 141 and 142: SP-1231 146 soils. In this approach
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Page 147 and 148: SP-1231 Fig. II.5.10.2/2. Enantiome
Page 149 and 150: SP-1231 154 References achiral colu
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Page 157 and 158: minerals or oxidat
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SCIENTIFIC METHOD AND REQUIREMENTS
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The sample materia
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surprising. To ach
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The main</
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Consideration of a diamond wire-saw
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Schoonen, M.A. & Barnes, H.L. (1991
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SP-1231 180 II.7.2 The</str
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SP-1231 Fig. II.7.4/1. Look
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SP-1231 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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